What you can't fully expect when you're expecting: Accurate gender

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An obstetric ultrasound of a fetus at the fourth month.

Story highlights

Depending on who is evaluating the ultrasound, they can be about 90% effective

Condoms are one of the least effective birth control methods, with a failure rate of 18%

(CNN)For expecting parents, 20 weeks is a milestone. It marks the halfway point of the 40 week pregnancy, but most excitingly it is the time many soon-to-be moms and dads find out the answer to the big question: boy or girl?

But what happens when instead of the little girl everyone has been expecting and buying clothes for a little boy ends up being born? Kyle and Danielle Williams captured their family's reaction on camera, and their reactions went viral.

How likely is this mix up? "It depends on how the baby is situated in the uterus," said sonographer Catherine E. Rienzo, a fellow with the Society of Diagnostic Medical Sonography. "Sometimes it's not that easy. Size of the uterus, abdominal scars, position of the baby and other factors that can play into it. If it's a male and the testicles haven't descended, it can look like a female. It's not 100%."

Making the wrong call happens more frequently than we realize, perhaps as high as one out of ten times. "It's not that uncommon to have gender wrong," said Dr. John Williams III, Director of Reproductive Genetics at Cedars-Sinai Medical Center, said. "It's just a screening tech. They can't make a guarantee of that. "

In addition, in recent years, more and more women are getting cell-free fetal DNA tests that are done to screen for genetic disorders like Trisomy 18 and Downs Syndrome, but can also determine gender. These tests are a simple blood draw, done around the ten week mark that can measure fetal DNA particles that make their way into the mother's blood. The tests are usually performed on women who are considered to be high-risk pregnancies. But Dr. Williams reiterated that these tests are not diagnostic.

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Overall, Williams said that the screening methods are fairly satisfying. "Cell-free DNA is probably 95% plus correct and ultrasound depends on who's doing it. But if it's done by a skilled person... there's 90% to 95% certainty on gender."

Pregnancy and birth control by the numbers

In fact, gender determination isn't the only thing surrounding fertility that isn't precise. Despite everyone's concerns about effectiveness, birth control methods can have a range of success rates. According to the Centers for Disease Control, intrauterine devices have a failure rate of between 0.2% and 0.8%, whereas hormonal methods like the traditional pill are effective about 9 out of 10 times. Barrier methods like condoms are in fact some of the least effective methods according to the CDC with a failure rate of 18%.

And if you do become pregnant, how accurate are your typical drugstore pregnancy tests? "Overall, they are pretty certain," said Williams. But, he still warns that their accuracy depends on human error. "There are false positives, false negatives. They frequently depend on the first morning urine, but if the patient has been drinking a lot of liquid -- it could dilute the hormone. And if it's too low on the urine test, it will be a false positive. Things happen." But he adds that today's tests are more sensitive than they were 30 years ago, when they first came on the market.

Is anything certain?

So when it comes to pregnancy, is anything certain? At least when it comes to gender identification, the only way to be certain according to both Dr. Williams and Riezno is to do Chornic Villi Sampling, known as CVS, and having an amniocentesis done. "The only way to do it -- is to do a diagnostic, with 99.9% certainty," said Dr. Williams. In these tests, cells are harvested from the placenta or the amniotic sac itself, and are then cultured to sequence the chromosomes. While much more invasive, it is also much more precise.

But these tests are not done just for gender identification. "Physicians would not do those scans just for that. They are looking for general abnormalities," said Rienzo.

So, has Rienzo ever made a mistake in her 35 year career? "I have made a mistake one time," she said, 18 years ago. "I did it on a friend of mine. She already had a girl, and I said that she was having another girl. And she actually had a boy. Obviously in the past 18 years, the technology and machines has improved. But yes, I made that mistake."